The present invention relates to a radiation curable coating composition for metal surfaces and to a process for preparing such coated metal surfaces.
It is known in the art to use conventional solvent-based, heat-curable coatings and inks on aluminum cans. There are conventional, solvent-based, thermally cured ink and coating systems that can withstand mechanical working and thermal cycling, after curing, without failure of the bond between the ink or coating and the metal surface. The metal working might involve, for example, with an aluminum can, a die- and/or spin-neck process, followed by pasteurization in hot water. The spin-neck and die-neck processes involve stretching and necking down of the can to allow for the use of less expensive, smaller diameter can tops. The coatings and inks must adhere to the cans throughout and after these processes and through subsequent pasteurization. While there are conventional, solvent-based, thermally cured ink and coating systems that are suitable for such applications, Federal Regulations inhibit their use because of their solvent content. Also, the cure speed for such coatings is undesirably slow.
Radiation-curable inks and coatings for aluminum cans have been developed and are available that are free of volatile organic solvents. For example, U.S. Pat. No. 3,912,670 to Huemmer et al., relates to radiation curable coating compositions for metal containers comprising a radiation curable oil, an optional radiation curable oligomer such as an acrylated or methacrylated epoxy compound, a flow control additive, and a reactive solvent which may be a mixture of acrylic acid and at least one other acrylic monomer, e.g., cyclohexyl acrylate. The acrylic acid is said to be critical in forming adhesion between the coating and the aluminum substrate.
U.S. Pat. No. 4,180,598 to Emmons relates to a radiation curable coating for metal (e.g., aluminum) substrates comprising 3-acyloxypropionic acid added to a composition comprising (a) at least one ethylenically unsaturated monomer such as isobornyl acrylate or dicyclopentenyl oxyethyl acrylate, (b) at least one oligomeric vinyl addition polymer of at least one monoethylenically unsaturated monomer having a terminal H.sub.2 C.dbd.C&lt; group, and (c) at least one acrylated oligomeric product having at least two acryloxy groups. The 3-acryloxypropionic acid is said to improve the adhesion of the cured coating to metal substrates.
It is also known to produce radiation curable primer coatings for metal substrates such as steel pipe. European Patent Application 0 177 791 A2 relates to a radiation curable primer coating composition which is applied to a steel substrate prior to application of a polyolefin coating. This primer coating composition comprises the reaction product of an epoxide with an ethylenically unsaturated monocarboxylic acid, a compound such as dicyclopentenyl acrylate, an epoxy compound, and optionally, an oligomer such as an unsaturated polyurethane such as is produced by reacting trimethylolpropane, tolylene diisocyanate, and hydroxyethyl acrylate.
However, the radiation curable can coatings that are known in the art are deficient in that, once cured on a metal substrate, they are not generally capable of withstanding working of the metal, such as would occur in shaping a can body by spin and die necking, especially where there is a subsequent heat treatment such as pasteurization in hot water. Prior art coatings have limited ductility, flexibility, and extensibility, and tend to shrink upon curing, so that the coating tends to part from the metal substrate.
It is also known in the art that, when radiation curable coatings are cured by ultraviolet light, it is desirable to use a combination of photoinitiators and a coinitiator. Especially effective coinitiators are amines.
Radiation curable coatings have been developed which are capable of withstanding metal working and pasteurization. Shustack, U.S. Ser. No. 134,975, filed Dec. 18, 1987, relates to a composition including at least a bulky monomer component, an oligomer component, and an acidic adhesion promoter. While such coatings are superior to prior art coatings, it has been observed that these acidic adhesion promoter-containing coatings must be formulated in the substantial absence of alkaline materials such as amines, inasmuch as alkaline materials tend to neutralize and render ineffective the acidic adhesion promoter, as well as to cause formulation instability (e.g., premature gelling). Therefore, these compositions may not include amine coinitiators.